The present invention relates to semiconductor laser devices, and more particularly relates to a semiconductor laser device used for an optical disc device such as a DVD-RAM, a DVD-R, a DVD-RW, a DVD+R, a DVD+RW, a CD-R, a CD-RW, a DVD-ROM, a CD-ROM, a DVD-Video, a CD-DA, a VCD or the like, optical information processing, optical communication, optical measurement or the like, and a method for fabricating such a device.
Semiconductor laser devices have been used for pickup light sources of optical disc devices and other light sources for optical information processing, optical communication and optical measurement. For example, an AlGaInP based red laser having a wavelength of 650 nm is used as a pickup light source for performing a read/write operation from/onto a DVD-RAM. As another example, an AlGaAs based infrared laser having a wavelength of 780 nm is used as a pickup light source for performing a read/write operation from/onto a CD-R.
To support both of DVD-RAM and CD-R, it is necessary to provide both of a red laser and an infrared laser in a single drive. Therefore, in general, drives including both of an optical integrated unit for DVD and an optical integrated unit for CD have been widely used. In recent years, however, because reduction in the size and cost of drives, simplification of assembling process steps for optical systems and the like are required, there are strong demands for development of drives which can support both of DVD-RAM and CD-R and include a single optical integrated unit.
For this reason, in recent years, double wavelength laser devices in which an AlGaInP based red laser having a wavelength of 650 nm and an AlGaAs based infrared laser having a wavelength of 780 nm are integrated are becoming put into practical use (see, for example, Japanese Laid-Open Publication No. 2001-57462). With use of a double wavelength laser device as a pickup light source, an optical disc device including a laser for both DVD and CD on a single optical integrated unit can be realized.
To perform a data write operation on a DVD-RAM, a CD-R or the like, increase in output power of a semiconductor laser device is required.
However, in a semiconductor laser device, an interface state is formed in part of an active layer located in the vicinity of a light emitting edge surface (this vicinity area will be hereafter referred to be as a “light emitting edge surface vicinity region”), so that non-radiative recombination of carriers is facilitated. Accordingly, in the part of the active layer located in the light emitting edge surface vicinity region, laser light generated in the semiconductor laser device is absorbed to generate heat, thus resulting in increase in temperature.
As an optical output density is increased, a larger amount of laser light is absorbed in the part of the active layer located in the light emitting edge surface vicinity region, so that increase in temperature due to heat generation is accelerated. As a result, finally, the temperature in the part of the active layer located in the light emitting edge surface vicinity region is increased to reach a melting point of a crystal constituting each semiconductor layer. Thus, part of each semiconductor layer located in the light emitting edge surface vicinity region is melted and a laser emission operation is terminated.
As described above, in the semiconductor laser device, the part of each semiconductor layer located in the light emitting edge surface vicinity region is damaged when the semiconductor laser device is in a high power density operation (hereafter, this will be referred to as “optical damage”).
As a solution for the above-described problem, semiconductor laser devices having a window structure have been put into practical use.
Hereafter, a method for fabricating a semiconductor laser device having a window structure will be simply described (see, for example, IEEE journal of Quantum Electronics, Vol. 29, No. 6, pp. 1874-1879 (1993)).
First, a plurality of semiconductor layers including an active layer having a multiple quantum well structure are formed on a substrate. Subsequently, a ZnO film is selectively formed over parts of the semiconductor layers located in the light emitting edge surface vicinity region by sputtering. Then, Zn contained in the ZnO film is diffused by annealing so as to reach lower part of the ZnO film in the active layer.
Thus, in part of the active layer in which Zn is diffused, a bandgap can be expanded by disordering the part of the active layer, so that a window region having a larger bandgap than a bandgap of an inner region in the active layer can be formed in the part of the active layer located in the light emitting edge surface vicinity region.
As described above, in the semiconductor laser device having a window structure, a window region having a larger bandgap than the bandgap of the inner region of the active layer is provided in the part of the active layer located in the light emitting edge surface vicinity region.
Thus, in the semiconductor laser device having a window structure, laser light generated in the semiconductor laser device is not absorbed in the part of the active layer located in the light emitting edge surface vicinity region, so that termination of a laser emission operation can be prevented.
As has been described, in a double wavelength laser device, to achieve an infrared laser and a red laser which allow high power operation, a window region has to be formed in each of part of an infrared laser active layer located in a light emitting edge surface vicinity region and part of a red laser active layer located in a light emitting edge surface vicinity region.
However, in a method for fabricating a double wavelength laser device, when Zn diffusion for forming a window region in an infrared laser and Zn diffusion for forming a window region in a red laser are performed in a single step, that is, a infrared laser window region and a red laser window region are formed by a single Zn diffusion step, the following problems arise.
As has been described, in a double wavelength laser device including an infrared laser and a red laser, AlGaAs mixed crystal is used as an infrared laser active layer and AlGaInP mixed crystal is used as a red laser active layer.
In this case, a Zn diffusion rate in AlGaAs mixed crystal is smaller than a Zn diffusion rate in AlGaInP mixed crystal.
Thus, when conditions for the Zn diffusion process step are adjusted so that a necessary amount of Zn for achieving the function of a window structure is diffused in the infrared laser active layer of AlGaAs mixed crystal, an excessive amount of Zn is diffused in the red laser active layer of AlGaInP mixed crystal.
Thus, crystal quality in part of the red laser semiconductor layer located in the light emitting edge surface vicinity region is drastically deteriorated. Furthermore, excessively diffused Zn in the part of the red laser semiconductor layer located in the light emitting edge surface vicinity region reaches even the substrate, thus resulting in electrical short circuit in the semiconductor laser device.
In contrary, when conditions for the Zn diffusion process step are adjusted so that a necessary amount of Zn for achieving the function of a window structure is diffused in the red laser active layer of AlGaInP mixed crystal, a sufficient amount of Zn for achieving the function of a window structure can not be diffused in the infrared laser active layer of AlGaAs mixed crystal.
Accordingly, a preferable window region can not be formed in the part of the infrared laser active layer located in the light emitting edge surface vicinity region, so that laser light generated in the semiconductor laser device is absorbed in the part of the infrared laser active layer located in the light emitting edge surface vicinity region. Thus, the part of the infrared laser semiconductor layer located in the light emitting edge surface vicinity region is melted, so that infrared laser emission operation is terminated.
As described above, in the method for fabricating a double wavelength laser device, a preferable window region can be formed in one of the infrared laser and the red laser but not in the other one by a single Zn diffusion step.
Therefore, in the method for fabricating a double wavelength laser device, when a Zn diffusion step of forming a window region in an infrared laser and a Zn diffusion step of forming a window region in a red laser are separately performed, that is, an infrared laser window region and a red laser window region are formed by separate two Zn diffusion steps, respectively, the following problems arise.
Since the method for fabricating a double wavelength laser device requires two separate Zn diffusion steps, the number of fabrication process steps is increased and thus fabrication cost is increased. Furthermore, in the double wavelength laser device, two separate Zn diffusion steps are performed, so that Zn diffusion step is performed twice to one of an infrared laser and a red laser. Therefore, a yield of the double wavelength laser device is reduced.